The present invention relates to an optical recording medium, and more particularly, to a printable optical recording medium which has a good surface printability and a high whiteness on a printing surface thereof, and is a high chroma of images printed thereon.
Meanwhile, the definition of "printable" or "printability" means a capability of writing or printing images such as characters or patterns by various writing utensils or printers.
Optical recording media, i.e., so-called optical disks, have been extensively used as ROM-type recording media for audio software, computer software, game software, electronic publishing or the like, because they have a large recording capacity and an ability of random access thereto as compared to conventional recording media. In addition, in recent years, write-once type recordable optical disks having an organic or inorganic recording layer on which information can be recorded based on various recording principles, have been developed and some of these optical disks have already been put into practice. One of these write-once type optical disks is known as a recordable compact disk (CD-WO). The recordable compact disks are not only recordable in a write-once manner, but also have a reflectance equal to those of ROM-type compact disks. Therefore, the recordable compact disks have such an advantage that information recorded thereon can be read out by a disk player or a driving unit ordinarily used for ROM-type compact disks.
Recently, the recordable compact disks have also been widely utilized by personal users. For example, amateur musicians, amateur players or computer users have used these recordable compact disks to record their own works and prepare self-making CD or CD-ROM. In addition, in various fields, these recordable compact disks have been used to record private information thereon.
When the optical recording media on which the private information has been recorded, are stored, maintained or distributed, it is necessary to indicate indices or other designs of the recording contents on a surface thereof. Conventional recordable compact disks which have been already put into practice or commercially available, have a protective layer as an outermost layer which is kept blank or on which merely common characters or patterns are printed with ultraviolet curable ink or oil-based ink. The blank or printed surface of the protective layer is, however, unsuitable for representations by ink or the like. Consequently, when users intends to print indices, etc. on surfaces of the recordable compact disks, the printing cannot be conducted by using a simply available apparatus such as ink-jet printers, thereby causing difficulty in freely printing private information thereon.
For this reason, in general, the private information has been manually written on the surface of each recordable compact disk by using an oil-based ink felt pen, etc., which is extremely bothersome because the writing work must be done in one-by-one manner. Further, there is caused such a problem that patterns written on the surfaces of the recordable compact disks or image qualities thereof cannot be uniformalized, thereby damaging the appearance of the optical recording media. On the other hand, labels or the like have been frequently adhered onto the surface of the recordable compact disk to indicate the private information thereon. However, in this case, there is still caused an inconvenience that the adhered label, etc., is projected by a height corresponding to its thickness from the surface of the disk, so that the optical recording medium suffers from not only inaccurate centering, deflection in plane or the like upon reading-out and recording, but also peeling-off or delamination of the protective layer together with the label when the label is removed from the surface thereof, thereby damaging the disks themselves.
In order to solve the afore-mentioned problems, there have been proposed a method of forming an ink-receiving layer composed of a hydrophilic resin on a surface of an optical recording medium (Japanese Patent Application Laid-Open (KOKAI) No. 6-60432(1994)), a method of improving an ink-fixing property of an optical recording medium by incorporating an organic or inorganic filler into a surface protective layer thereof (Japanese Patent Application Laid-Open (KOKAI) No. 7-169100(1995): U.S. Pat. No. 5,573,831), or the like.
Especially, Japanese Patent Application Laid-Open (KOKAI) No. 7-169100(1995) teaches an optical recording medium comprising:
a transparent substrate;
a recording layer on the transparent substrate;
a metal reflective layer; and
at least one protective layer,
these layers being laminated successively on the transparent substrate, the outermost protective layer being formed of an ultraviolet curable resin containing an organic filler and/or an inorganic filler having water absorbency and/or oil absorbency, so that the protective layer has a printable surface, and the outermost protective layer formed of the ultraviolet curable resin being formed by means of curing with an ultraviolet radiation an ink obtained as a mixture of a filler and an ultraviolet curable resin composition comprising:
a) from 1 to 80 parts by weight of hydrophilic polymer;
b) from 20 to 98 parts by weight of hydrophilic monomer;
c) from 0 to 40 parts by weight of cross-linking monomer; and
d) from 0.1 to 10 parts by weight of radical initiator.
Although it is described in the said Japanese Patent KOKAI that the protective layer is free from ink repellency and can show an excellent fixation property for printing ink by using the hydrophilic polymer, the protective layer of the afore-mentioned optical recording medium is unsatisfactory in blur for printing ink.
In addition, since the protective layer contains the organic or inorganic filler, there arises a problem that not only a color tone of the reflective layer but also a color tone of the filler adversely affects clarity or definition of printed images such as characters or patterns.
On the other hand, in general, gold has been used as a material for the reflective layer, because a color tone of the gold well matches that of a specific organic pigment contained in a light-absorbing layer (a recording layer) and the gold can show a high reflectance to a laser beam used for recording and reading-out of the optical recording medium. However, in the case where the gold is used as a material of the reflective layer of the afore-mentioned optical recording media of which ink-fixing property has been improved, since the surface of the ink-receiving layer thereof is colored golden, there arises another problem that the clarity or definition of printed characters or patterns, especially that of yellow color portions, is seriously damaged.
Also, as the method for improving a color tone on the printing surface of the optical recording medium, there is proposed a method of providing a white pigment layer containing titanium white or the like between the protective layer and the ink-receiving layer. However, the problem that the printing surface is undesirably colored due to the filler contained still remains unsolved even though the white pigment layer is formed. Further, since the particles of the pigment is hard, there is caused a risk of damaging the recording layer when the characters or patterns are written on the surface of the optical recording medium using writing utensils having a hard tip.
As a result of the present inventors' earnest studies to solve the afore-mentioned problems, it has been found that in an optical recording medium comprising a transparent substrate and at least a recording layer, a reflective layer and a protective layer, an ink-receiving layer or a protective and an ink-receiving layer, which layers are in order superposed on the transparent substrate, by (a) incorporating a hydrophilic and water-insoluble filler having a Mohs hardness of not more than 3 into the protective layer or the ink-receiving layer, (b) adjusting an arithmetic mean surface roughness Ra on the surface of the layer to 0.3 to 2.0 .mu.m and (c) adjusting a "b" value in Lab color space as a chromaticity of surface of the layer to +5 to -5, characters or patterns can be readily and uniformly written or printed on the surface of the resultant optical recording medium with a high chroma. The present invention has been attained on the basis of this finding.